There are commercial optical disks called DVD and BD as high-density, large-capacity optical information recording media. Such optical disks are being rapidly spread recently as recording media for recording images, music, and computer data. An optical disk having a plurality of recording layers as shown in Japanese Unexamined Patent Publication No. 2001-155380 to further increase the recording capacity has been also proposed.
FIGS. 19 and 20 show the configuration of a conventional optical recording medium and a conventional optical pickup.
First, FIG. 19 will be described. An optical recording medium 401 in FIG. 19 has information recording faces 401a and 401b. Thickness d1 of a protection layer from the surface of the information recording face 401a is 0.075 mm, and thickness d2 of a protection layer from the surface of the information recording face 401b is 0.1 mm.
The case of recording/reproducing information to/from the information recording face 401b will be described.
A light source 1 such as a semiconductor laser emits a linearly polarized beam 70 having a wavelength λ1 of 405 nm. The diverging beam 70 emitted from the light source 1 passes through a collimate lens 53 having spherical aberration correcting units 93 and whose focal length f1 is 15 mm and enters a polarization beam splitter 52. The beam 70 which has entered the polarization beam splitter 52 passes through the polarization beam splitter 52 and passes through a quarter-wave plate 54 where it is transformed to circularly polarized light. After that, the circularly polarized light is converted to a converged beam by an objective lens 56 having a focal length f2 of 2 mm. The converged beam passes through a transparent substrate of the optical recording medium 401 and is condensed onto the information recording face 401b. In FIG. 19, the position of the collimate lens 53 is controlled by the spherical aberration correcting units 93 so that spherical aberration becomes almost 0 mλ in the information recording face 401b. The opening of the objective lens 56 is regulated by an aperture 55, and numerical aperture NA is 0.85. The beam 70 reflected by the information recording face 40b passes through the objective lens 56 and the quarter-wave plate 54 and is transformed to a linearly polarized beam which is different from the incoming linearly polarized beam by 90 degrees, and is reflected by the polarization beam splitter 52. The beam 70 reflected by the polarization beam splitter 52 passes through a condenser lens 59 whose focal length f3 is 30 mm and is transformed to a converged beam. The converged beam passes through a cylindrical lens 57 and enters a photodetector 32. Astigmatism is given to the beam 70 when it passes through the cylindrical lens 57.
The photodetector 32 has four light receiving parts (not shown). The light receiving parts output current signals I30a to I30d according to received light amounts, respectively.
A focus error (hereinbelow, called FE) signal caused by the astigmation method is obtained by (I30a+I30c)−(I30b+I30d). A tracking error (hereinbelow, called TE) signal in the push-pull method is obtained by (I30a+I30d)−(I30b+I30c). An information (hereinbelow RF) signal recorded on the recording medium 401 is obtained by I30a+I30b+I30c+I30d. The FE signal and the TE signal are subjected to amplification and phase-compensation to a desired level, and the resultant signals are supplied to actuators 91 and 92 and subjected to focus and tracking controls.
In the case of recording/reproducing information to/from the information recording face 401b, the position of the collimate lens 53 is controlled by the spherical aberration correcting units 93 so that spherical aberration becomes almost 0 mλ in the information recording face 401a. 
Next, FIG. 20 will be described. FIG. 20 shows the configuration of an optical pickup having a configuration similar to that of FIG. 19. The diverging beam 70 emitted from the light source 1 passes through the collimate lens 53 having spherical aberration correcting units 93 and whose focal length f1 is 15 mm and enters the polarization beam splitter 52. The beam 70 which has entered the polarization beam splitter 52 passes through the polarization beam splitter 52 and passes through the quarter-wave plate 54 where it is transformed to circularly polarized light. After that, the circularly polarized light is converted to a converged beam by the objective lens 56 having the focal length f2 of 2 mm. The converged beam passes through the transparent substrate of the optical recording medium 401 and is condensed onto any one of recording layers 401a, 401b, 401c, and 401d formed in the optical recording medium 401. The objective lens 56 is designed so that spherical aberration becomes zero in an intermediate depth position between the recording layers 401a and 401d. Spherical aberration which occurs in the case where the beam is condensed to any of the recording layers 401a to 401d is eliminated by moving the position of the collimate lens 53 in the optical axis direction by the spherical aberration correcting units 93.
The opening of the objective lens 56 is regulated by the aperture 55, and numerical aperture NA is set as 0.85. The beam 70 reflected by the recording layer 401d passes through the objective lens 56 and the quarter-wave plate 54 and is transformed to a linearly polarized beam which is different from the incoming linearly polarized beam by 90 degrees, and is reflected by the polarization beam splitter 52. The beam 70 reflected by the polarization beam splitter 52 passes through the condenser lens 59 whose focal length f3 is 30 mm and is transformed to a converged beam. The converged beam passes through the cylindrical lens 57 and enters the photodetector 32. Astigmatism is given to the beam 70 when it passes through the cylindrical lens 57.
The photodetector 32 has not-shown four light receiving parts which output current signals according to received light amounts. From the current signals, a focus error (hereinbelow, called FE) signal caused by the astigmation method, a tracking error (hereinbelow, called TE) signal caused by the push-pull method, and an information (hereinbelow RF) signal recorded on the recording medium 401 are generated. The FE signal and the TE signal are subjected to amplification and phase-compensation to a desired level, and the resultant signals are supplied to the actuators 91 and 92 and subjected to focus and tracking controls.
Distance d1 from the surface of the optical recording medium 401 to the recording layer 401a, distance d2 from the recording layer 401a to the recording layer 401b, distance d3 from the recording layer 401b to the recording layer 401c, and distance d4 from the recording layer 401c to the recording layer 401d are set so that their ratios are d1:d2:d3:d4=2:3:4:5. The reason why the distances d1 to d4 are not set to the same distances but are set at the ratios will be described below.
If the distances d1 to d4 are the same, the following problem occurs. For example, when the beam 70 is condensed to the recording layer 401d to record/reproduce information to/from the recording layer 401d, part of the beam 70 is reflected by the recording layer 401c. Since the distance from the recording layer 401c to the recording layer 401d and the distance from the recording layer 401c to the recording layer 401b are the same, part of the beam 70 reflected by the recording layer 401c forms an image on the back side of the recording layer 401b. The reflected beam is reflected again by the recording layer 401c and mixed with reflected light from the recording layer 401d from which information is to be inherently read. Further, since the distance between the recording layer 401b to the recording layer 401d and the distance from the recording layer 401b to the surface 401z of the optical recording medium 401 are the same, part of the beam 70 reflected by the recording layer 401b forms an image on the back side of the optical recording medium 401. The reflected beam is reflected again by the recording layer 401b and mixed with reflected light from the recording layer 401d from which information is to be inherently read. The reflection light that forms an image on the back side of another layer overlaps and is mixed with reflection light from the recording layer 401d from which information is to be inherently read. A problem occurs such that the mixed light hinders recording/reproduction.
To prevent the problem, a method is disclosed in which the distances between the recording layers are set so as to become gradually longer from the surface 401z of the optical recording medium 401. When the beam 70 is condensed to the recording layer 401d from which information is to be inherently read, images are not formed simultaneously on the back side of the recording layer 401b and the back side of the surface 401z (refer to Japanese Unexamined Patent Publication No. 2001-155380). Each of the distances d1 to d4 has manufacture variations of ±10 μm. Since the distances d1 to d4 have to be set so as to be different from each other even in the case where the distances d1 to d4 vary, the difference between the distances is set to, for example, 20 μm. In this case, d1=40 μm, d2=60 μm, d3=80 μm, and d4=100 μm. Total interlayer distance d (=d2+d3+d4) from the recording layer 401a to the recording layer 401d is 240 μm.
To realize larger capacity, it is considered to increase the number of multiple layers of the recording layer. In the case of an optical recording medium having a plurality of recording layers, if the interlayer distances of signal faces (signal face distances) including variations are the same, due to the influence of a signal face different from a signal face from which information is to be inherently read, it is difficult to read a stable signal. For example, in the case of an optical disk having four signal faces, at the time of reading a signal from the fourth face, the optical pickup is focus-controlled so that focus is achieved on the fourth face. However, a part of the beam is reflected by the third layer and focus is achieved on the signal face of the second layer. The beam reflected by the second layer in which focus is achieved is again reflected by the third layer, and the reflected beam travels in the same optical path as that of the beam reflected by the fourth face and enters a detector of the optical pickup. The beam reflected by the second layer exerts an adverse influence on a signal of the fourth face from which a signal is to be inherently read.
To solve the problem, Japanese Unexamined Patent Publication No. 2001-155380 presents a technique of reading a more stable signal by changing the thickness of a base material between neighboring information recording faces. In practice, however, a stable signal cannot be read only by changing the thickness of the base material. More specifically, in the conventional configuration, by making the values of the interlayer distances d1 to d4 different from each other, mixture of reflection light from a recording layer to/from which information is recorded/reproduced with reflection light whose focus is achieved on the back side of another layer can be prevented. When the distance between layers is set in consideration of even variations of the distance between layers, there is a case that the total interlayer distance d (=d2+d3+d4) becomes extremely large. When the total interlayer distance d increases, the absolute amount of spherical aberration to be corrected increases, and a problem occurs such that remaining aberration which cannot be removed by the spherical aberration correcting units 93 also increases. When the total interlayer distance d is small, the spherical aberration correcting units 93 can remove the spherical aberration almost perfectly. However, when the total interlayer distance d increases and the absolute value of the spherical aberration increases, jitter becomes worse due to the remaining aberration which cannot be removed, and the adverse influence is exerted on recording/reproduction.